I recently installed a home battery system, intended to be act 1 of a solar install due to occur next year. At the moment no solar is installed, but the battery can still charge on cheap overnight rates, discharging during the peak hours. With that, lets review my electricity tariff (Ultralow overnight rate), and some usage strategies. Ill try to keep the discussion independent of my battery vendor (Tesla powerwall 3 for the record).
Lets begin with the posted rates from Toronto Hydro (source), for the ultralow overnight rate, lets call that the “sell-price” for reasons we will explore later. Next we add the 2.5 c/kWh transmission charge to arrive at what we call the “buy-price”. We do this mostly as when you get a kWh from the grid, the grid will charge us for the power as well as the transmission charge. However, if you happen to have permission from your utility to export energy to the grid, net metering suggests, your credit would be just the cost of power component without the transmission fee.
| Time period | Sell Price [c/kWh] | Buy Price [c/kWh] |
| Peak (4pm-9pm, weekdays) | 28.4 | 30.9 |
| Mid-Peak (7am – 4pm, and 9pm – 11 pm weekdays) | 12.2 | 14.7 |
| Off-peak (weekends 7 am – 11 pm) | 7.6 | 10.1 |
| Overnight (11pm-7am) | 2.8 | 5.3 |
Next, we need to determine how much of the battery we want to reserve for backup reasons. This is often an individual choice, and comes down to your power goals. For me, I went with the recommended 20% reserve. That seemed reasonable, experience has shown that 20% will run our house 1-3 hours depending on usage, and where I live, power outages while they do happen, are rarely longer than an hour.
Depending on your battery system software, that might be more or less it, but lets discuss some usage strategies. At a minimum, I would expect a home battery system to get you through the expensive peak hours. I have also found that the powerwall 3, with its 13.5 kWh goes from 100% charged to my 20 % reserve limit, in 8-16 hours depending on usage. That’s quite a bit more than the 5 hours I need to cover the peak, and can thus help me offset some of my mid-peak daytime usage as well. On a typical day, the powerwall 3 will start powering the house at about lunch (12-1 pm), and keeps going for the next 10-11 hours until the overnight rate begins at 11 pm.
Electric car adventures 